In subscription television systems, a television signal is typically transmitted, either "over-the-air" or through a suitable cable network, in a scrambled, that is, encoded form to render the video information largely unviewable when the signal is received and processed by a conventional television receiver. Authorized subscribers are provided with decoders that unscramble the scrambled signals to render them viewable on the television receiver.
A common signal scrambling technique is horizontal sync suppression, which causes erratic operation of the receiver deflection circuits and results in a scrambled display. In addition, sync suppression may adversely affect recovery of the color reference burst and cause inaccurate color reproduction.
The sync suppression system disclosed in U.S. Pat. No. 4,467,353 to Richard W. Citta and Ronald B. Lee provides reliable and secure television signal encoding and decoding. In this system, in addition to suppression of the horizontal sync components, the phase of the video signal carrier is reversed during the horizontal blanking interval. These phase reversals are detected in the decoder for enabling restoration of the horizontal blanking intervals and synchronization pulses to their proper levels.
U.S. Pat. No. 4,706,283, in the names of Richard W. Citta, Dennis M. Mutzabaugh and Gary J. Sgrignoli, incorporated herein by reference, discloses apparatus and methods using surface acoustic wave ("SAW") filters in a television scrambling system employing horizontal sync pulse suppression and carrier phase reversal. The stable frequency response and attenuation characteristics of SAW filters provide reliable signal phase shifting and attenuation during the horizontal blanking intervals. Similarly, the use of complementary SAW filters in the decoder provides for reliable restoration of the horizontal blanking intervals and syncs in the received signal.
The referenced systems encode the video signal by applying it to a pair of SAW filters having differing phase and amplitude attenuation characteristics. A timing pulse, developed from the baseband composite video signal, controls a switch, that in turn controls which of the pair of SAW filters is coupled to the output circuitry. The output circuitry includes a vestigial side band filter required in the NTSC system. The vestigial side band filter is non-linear and introduces non-linear attenuation and phase changes to the video carrier.
The decoder has a pair of SAW filters, which are complementary to the pair of SAW filters in the encoder. The decoder also includes a synchronous detector for generating timing pulses which operate a switch for selecting the outputs of the SAW filters in a manner so as to decode the received signal.
In this system, both the endcoding and decoding SAW filters are switched at a very rapid rate, typically on the order of 1 to 2 nanoseconds which represents a frequency that is much higher than the video carrier frequency. In the encoder, these high switching rates of the RF video signal result in the transmission of single-sideband transients due to the effect of the vestigial sideband filter. These transients can adversely affect the operation of the decoder as well as that of the television receiver connected to the output of the decoder.
In an effort to reduce switching transients, some prior art sync suppression systems reduce the amplitude of the picture carrier during switching times. In order to remain in the double-sideband region of the vestigial sidebannd filter, such amplitude reductions must be maintained over relatively long periods of time. This, however, introduces other problems particularly in television receiver circuits requiring a continuous video carrier, such as intercarrier sound circuits and synchronous detectors.